The production of energy by photovoltaic means can only be considered for replacing conventional energy sources which are in many regards disadvantageous when the production costs of solar cells is reduced. Therefore, it is necessary to reduce the cost of every part of the production process form the preparation of the basic material to the finished solar cell. One critical process step is the step in which the basic material, mostly in bar or block form, is sawed to form discs from which the solar cells are made.
The sawing is generally done by two methods: the inner-hole sawing method, wherein the bar is sawed disc by disc, or the multiple lap cutting method wherein the bar is simultaneously divided into a multiplicity of discs.
Multiple lap cutting processes are known, especially for the cutting of silicon or germanium blocks, from German DE-OS No. 20 39 699 and DE-OS No. 27 22 782. In the publications, the blocks under pressure are cut downwardly from above by means of a saw frame reciprocatingly moved at a certain lateral speed. During the cutting operation, a suspension of lapping abrasive is fed from the top of the blades. The cutting suspension can be prepared according to DE-OS No. 27 22 780. The suspension penetrates into the dividing gap, reaching the cutting zone, and finally flowing downwardly, carrying the eroded material away from the cutting zone. However, the discs obtained by these processes, which are capable of high production rates, include, in many cases, fine cracks and cannot be used. Additional process difficulties are caused in the case of workpieces of rectangular cross section, by the impulse attributed to the uneven erosion of the blades, which impulse bears upon the workpiece at the reversal point of the saw frame, and can result in damage such as marginal ruptures (see German patent application P No. 32 47 836.7). In such case, the damaged discs cannot be used.
DDR commercial Pat. No. 67,331, publication date June 5, 1969, discloses a lap cutting process which is stated to improve the surface quality of the discs being cut off by forcing an additional rotating motion upon a workpiece that is immersed in a tub filled with an emulsion of lapping abrasive and reciprocatingly moved over the cutting blades mounted therein. The emulsion of the lapping abrasive covers the cutting blades in the cutting zone of the semiconductive crystal bar, and brings about an improvement in the cutting quality.
The object of the present invention, on the other hand, is to provide a process which reduces losses in yield resulting from mechanical damage to the discs during the cutting operation when cutting solid materials, especially mono- or polycrystalline blocks of semiconductive material such as silicon or germanium.